Automotive heating, air conditioning and ventilation systems (HVAC systems) typically incorporate a large, hollow housing behind the instrument panel, which contain a cold evaporator core and hot heater core, as well as various valves for directing air flow through them and ultimately out into the passenger compartment. Before the air enters the passenger compartment, a temperature valve divides and directs the unconditioned outside air flow through the cold and hot cores in a relative proportion designed to attain a selected final temperature, after which the air flow is directed back to a consolidated mix area where they are blended and mixed. From the mixing area, a so called mode valve directs the tempered and conditioned air flow into one or more outlets into the passenger compartment.
When the system is designed to automatically attain a final air temperature selected by a vehicle occupant, sensors in the various outlet ducts continually monitor the discharge temperature and adjust the temperature valve setting accordingly. An individual sensor and its associated wiring is needed for every duct which exits the housing and must be individually handled and routed with harnesses and connectors. In addition, since the duct sensors are located significantly downstream from the tempered air mixing area, there is an inevitable time and distance lag between air leaving the mix area and entering the selected duct or ducts. In addition, when air flow is being switched into a duct that previously had no air flow, its sensor is initially reading stagnant air before tempered air flow eventually reaches it.